1. Field of the Invention
The present invention relates to a solid state image pickup device.
2. Description of the Prior Art
A predetermined number of photoelectric converting elements, each of which is constructed of a photodiode and a MOS transistor using the photodiode as its source portion, are arranged to respectively correspond to picture cells, and the photoelectric converting elements, which are arranged in a row vertically of an image pickup frame, are generalized by a signal line and are connected each row with an output terminal through a switching MOS transistor. There are additionally provided horizontal and vertical scanning circuits which are constructed of MOS shift registers. The horizontal scanning circuit controls the gates of the MOS transistors through a signal line thereby to perform the horizontal scanning operation. The vertical scanning circuit generalizes and directly controls the gates of the MOS transistors of the respective photoelectric converting elements, which are arranged in a line horizontally of the image pickup frame, for each line thereby to perform the vertical scanning operation. The following description is directed to an n-channel type device using electrons as its signal charge, but a completely similar description can be applied to a p-channel type device using positive holes as its signal charge by inverting the conduction type and the polarity.
The electrons generated by a light are stored in the junction capacitors of the photodiodes. Upon the reading operation, the MOS transistors are rendered conductive through the signal lines by the positive scanning pulses which are generated by the vertical scanning circuit, and the MOS transistors are consecutively rendered conductive through the signal lines by the positive scanning pulses which are generated by the horizontal scanning circuit. Thus for the time period after the once scanning operation and before the subsequent scanning operation, the charges, which have been converted by the photodiodes from the light incident upon the picture cells and which have been stored in the junction capacitors of the photodiodes, are consecutively read out as signal charges.
In the image pickup device to be described, a parasitic capacitance exists between the gate and drain of the switching MOS transistor. Through that capacitance, the scanning pulses from the scanning circuits appear in the form of a spike noise. The intensity of this spike noise disperses due to irregularities in the threshold voltages of the transistors and in the capacitances. This results in establishment of the noise which has such a pattern as is fixed horizontally of the image to be picked up. In the solid state image pickup device or the image pickup tube, since the capacitance to appear at the output terminal generally has a value as high as about 10 pF, the high-frequency component of the image information leaks through that capacitance to the substrate. In order to compensate for this leakage, it is necessary to raise the high-frequency gain of a preamplifier of a camera to be connected with the output terminal of the image pickup device. On the other hand, since an element to be used as the input terminal of the preamplifier, e.g., a junction field effect transistor generates a white noise having an intensity distribution having no relationship with the frequency, the high-frequency noise is highly amplified, if the high-frequency gain of the preamplifier is raised, so that the S/N ratio of the device is deteriorated.
The image pickup device has an additional major problem. This is the phenomenon of blooming, in which a vertical white band appears from the brighter portion of the image pickup frame when a strongly lit image is picked up.